1. Field of the Invention
The present invention relates to a radiation image pick-up device, a radiation image pick-up method and a radiation image pick-up system which correct a radiographed object image.
2. Related Background Art
Currently, X-ray image pick-up devices used for medical diagnosis mainly use a so-called film system in which an X-ray is emitted to a human body, the X-ray having passed through the human body is applied to a scintillator for converting the X-ray into a visible radiation, and a fluorescence is exposed to a film.
However, more efficient diagnosis and more accurate medical devices are strongly demanded in hospitals of the world as well as in Japan which is becoming an aging society. Under such circumstances, because of a film development process of X-ray image pick-up devices using the conventional film system, it takes a long time before a doctor obtains an X-ray image of a patient. When a patient moves during X-ray radiographing or exposure is not correct, radiographing has to be performed again. These factors interfere with higher efficiency of diagnosis in hospitals and put a heavy burden on patients, resulting in a serious difficulty in establishing a new medical society for the future.
In recent years, the “digitalization of X-ray image information” has grown in demand among health care providers. If digitalization is achieved, doctors can obtain X-ray image information on patients with optimum angles in real time and the obtained X-ray image information can be recorded and managed using media such as a magneto-optical disc. By using networks and other communication systems, X-ray image information on patients can be transmitted to any hospital of the world in a short time. In order to meet the needs for the “digitalization of X-ray image information”, X-ray image pick-up devices using CCD solid state imaging devices and amorphous silicon photoelectric conversion elements instead of a film have been proposed in recent years.
FIG. 8 is a schematic sectional view showing a digital X-ray image pick-up device, which has been developed in recent years. The following will briefly describe the configuration of the device (e.g., Japanese Patent Application Laid-Open No. H08-116044).
Photoelectric conversion elements 21 using amorphous silicon and switching TFTs 22 are formed on a glass substrate 20. A protective layer 27 made of silicon nitride or the like covers the entire substrate to protect the elements. A reading circuit 28 for drawing electrical signals from the photoelectric conversion elements 21 to the outside (outside the substrate) and a shift register (not shown) for driving the TFTs are connected to the outer periphery of the glass substrate. The overall configuration serves as a photodetector array 8. A fluorescent screen 142 for converting a wavelength from an X-ray to a visible radiation is formed on the upper part of the photodetector array 8 by using a method such as bonding, so that the digital X-ray image pick-up device is finished.
As shown in FIG. 8, in the case of the digital X-ray image pick-up device, an X-ray 29 is incident from a point above the fluorescent screen 142, the X-ray 29 is subjected to wavelength conversion into a visible radiation 30 through the fluorescent screen 142, and the converted visible radiation 30 is detected by the photoelectric conversion element 21. Further, the detected light is converted into an analog electrical signal 80 and the signal is sequentially drawn to the reading circuit 28 by turning on/off the TFTs 22. Thereafter, the signal is converted into a digital signal 42 by an AD converter 40 which is provided in the subsequent stage of the reading circuit 28. The converted digital signal 42 is transferred to the image processing unit 10 and is subjected to image processing such as offset correction and gain correction, and the signal is displayed on a display 160, and then a diagnosis is performed by a doctor 106.
The following will describe actual radiographing using the digital X-ray image pick-up device.
FIG. 9 is a flowchart from the installation of the digital X-ray image pick-up device in a hospital to actual radiographing performed on a patient.
The digital X-ray image pick-up device having been carried out from a factory is subjected to several adjustments after being installed in a hospital. Calibration is always performed before actual radiographing. Calibration means radiographing performed by applying an X-ray with no radiographed object between an X-ray generator and the digital X-ray image pick-up device. Calibration data (hereinafter, referred to as a white image) serves as gain correction data when an object is actually radiographed. Thus, a radiographed white image is stored in a memory device 53 or the like and is read for gain correction every time radiographing is performed.
In the case of the digital X-ray image pick-up device using photoelectric conversion elements of amorphous silicon or the like, it is necessary to consider the correction of variations in sensitivity for each of the photoelectric conversion elements, variations in gain in the reading circuit, and the shading of the fluorescent screen and an X-ray. Thus, it is necessary to divide an object radiograph image by a white image. This processing is called white correction. Conventionally, a white image is radiographed under a certain condition (X-ray tube voltage/X-ray tube current/X-ray exposure time/X-ray tube vessel—a distance between digital X-ray image pick-up devices) and the image is used over a week, a month and a year. During this period, normal radiographing 52 of FIG. 9 is repeated.
After calibration, object (patient) information and radiographed portion information are inputted, radiographing conditions are determined, and radiographing is actually performed. At this point, the radiographing conditions are varied according to a radiographed portion and a thickness of an object. For example, a tube voltage (X-ray energy) and so on are adjusted in kilovolts. For this reason, the radiographing conditions of a white image and the radiographing conditions of an object do not match with one another. After radiographing, correction is performed in which a white image is read from the memory device 53, and white correction is performed on an object image in an image processing unit, and then the image is shown on a display.
The X-ray generator will be briefly described below because the present invention closely relates to X-ray absorption.
FIG. 10 is a schematic sectional view showing an X-ray tube which serves as an X-ray source in the X-ray generator.
Vacuum is almost maintained in the X-ray tube. By applying a voltage (several tens kV) between an anode and a cathode, electrons are accelerated from the cathode to the anode and collide with a target, so that an X-ray is generated. When an X-ray is generated by the X-ray generator, a tube voltage and a tube current are mainly adjusted. The tube voltage indicates a voltage applied to the cathode and the anode. As the tube voltage increases, the acceleration of electrons increases, thereby increasing the “energy” of the electrons. Further, the tube current indicates a current applied to a filament. As the tube current increases, the “number” of electrons outputted from the filament increases, thereby increasing the intensity of an X-ray. Therefore, although the intensity (number/amount) of an X-ray is increased by changing the tube current, “energy” does not increase.
An X-ray has various interactions (Rayleigh scattering, a photoelectric effect and so on) when passing through a substance. The way to interact varies according to the “energy” of an X-ray. Hence, the energy of a generated X-ray is varied by changing the tube voltage of the X-ray generator. Further, the interaction (amount of absorption/amount of transmission) with a substance is also changed.
The following will describe a scintillator used in the digital X-ray image pick-up device.
Currently, a main scintillator is Gd2O2S:Tb3+, CsI:Tl and so on. Gd2O2S:Tb3+ is prepared by coating/drying a scintillator and a binder resin on a PET (polyethylene terephthalate) sheet or the like. Although Gd2O2S:Tb3+ can be mass-manufactured and is inexpensive, a large amount of light is scattered because Gd2O2S:Tb3+ is a particulate scintillator, resulting in a low resolution. In contrast, CsI:Tl is a scintillator of a columnar structure and thus causes less scattered light with a higher resolution as compared with Gd2O2S:Tb3+. Further, a high brightness can be obtained by a large thickness. For this reason, CsI:Tl is widely used at present as a scintillator of the digital X-ray image pick-up device. Regarding CsI, Tl and Na are mainly used as activators at present. The activator is not particularly limited and thus will be referred to as CsI.
The following will describe the configuration and manufacturing method of CsI.
FIG. 11 is a schematic sectional view showing a CsI fluorescent screen. An X-ray emitted from a substrate 81 passes through the substrate 81 and is absorbed by a scintillator 82, and then wavelength conversion is performed from an X-ray to a visible radiation. Then, the visible radiation having undergone wavelength conversion passes through a protective layer 83 and is detected by a photodetector array which adheres to the protective layer 83. Thus, each material has to be characterized as follows:
[1] Substrate 81: small X-ray absorption
[2] Scintillator 82: large X-ray absorption with a high brightness and a high resolution
[3] Protective layer 83: a high transmittance of a visible radiation
X-ray absorption (transmittance) is determined by the attenuation coefficient and thickness of a material. The smaller atomic number of a material, the smaller attenuation coefficient, resulting in difficulty in absorbing an X-ray (higher transmittance). Moreover, X-ray absorption varies according to a tube voltage (X-ray energy). In general, as energy decreases, absorption increases.
FIG. 12 is a characteristic diagram showing a tube voltage and an X-ray transmittance of glass (85), aluminum (86) and amorphous carbon (87) which are mainly used as base materials. The amorphous carbon 87 has a small atomic number (z=6) and has a small amount of X-ray absorption particularly at a low energy. Thus, the amorphous carbon 87 has a high sensitivity at a low energy as compared with other base material.
A scintillator absorbs an X-ray and emits a visible radiation according to an amount of X-ray absorption. Like the base materials, the X-ray absorption of the scintillator is determined by a material and a thickness. Currently, CsI with a thickness of 500 μm is used most frequently. FIG. 13 is a characteristic diagram showing an X-ray absorptivity in CsI having a thickness of 100 to 500 μm.
The tube voltage characteristic of an X-ray in the scintillator and the base materials is determined by the kind of material and a thickness thereof and thus the tube voltage is changed by an impurity and a foreign matter or variations in thickness. Particularly in the event of a foreign matter and a partially uneven thickness, a tube voltage characteristic is changed only on that portion.
Generally CsI is formed on a base material by vacuum deposition shown in FIG. 14. The substrate 81 is fit into a substrate holder 88 which is mounted in the upper part of a chamber, CsI powder is put in a port 89 which is mounted in the lower part, and the port 89 is heated, so that CsI is evaporated to form the substrate 81 (resistance heating). Since CsI is a columnar crystal, a column vertically stretches from the substrate 81 under normal conditions. During deposition, CsI powder-dissolves and bumping occurs, resulting in a number of asperities called a splash 90 on a surface of CsI.
The digital X-ray image pick-up device has a large fluorescent screen formed on a large (e.g., 45×45 cm) photodetector array. Hence, it is difficult to prevent the entry of a foreign matter in a process of manufacturing a photodetector array and a fluorescent screen. As a matter of course, the substrate is cleaned particularly in the vapor deposition of CsI. However, the adhesion of only a small foreign matter causes abnormal growth of CsI with the foreign matter acting as a nucleus. FIGS. 15A to 15D are diagrams showing a state of abnormal growth of CsI used as a scintillator (wavelength converter).
FIG. 15A is a schematic diagram showing a state of abnormal growth of CsI used as a scintillator. Since CsI 82 of FIG. 15A is a columnar crystal under normal conditions, the CsI 82 vertically grows from an evaporation surface. When the evaporation surface has a foreign matter 91, the CsI 82 grows diagonally with the foreign matter 91 acting as a nucleus and is changed into asperities several times larger than the nucleus. The asperities (abnormal growth portion) change the thickness of the CsI 82 from a normal portion, so that X-ray absorption is also changed. Further, a tube voltage characteristic is also changed according to a thickness of CsI and thus a ratio of light emission differs between the normal portion and the abnormal growth portion.
FIGS. 15B to 15D are characteristic diagrams showing an amount of light emission in the normal portion and the abnormal growth portion. FIG. 15B shows that the abnormal growth portion and the normal portion are radiographed at a tube voltage of 80 kV. FIG. 15C shows that the abnormal growth portion and the normal portion are radiographed at a tube voltage of 60 kV. FIG. 15D shows a ratio determined by dividing the characteristic of FIG. 15C and the characteristic of FIG. 15B. This division means white correction. In the white correction, division is actually performed using images of different radiographing tube voltages.
As shown in FIG. 15B, the normal portion has an output of 100 in the radiographing at 80 kV, whereas the abnormal growth portion has a smaller output of 80. As shown in FIG. 15C, the normal portion has an output of 100 in the radiographing at 60 kV, whereas the abnormal growth portion has a smaller output of 90. In this way, a change in tube voltage changes a reduction in the output of the abnormal growth portion. This is because the thickness of CsI differs between the abnormal growth portion and the normal portion and thus X-ray absorption is changed.
In the case of the abnormal growth caused by the foreign matter of FIG. 15A, the X-ray absorption of the foreign matter also makes a major contribution. An amount of X-ray reaching CsI decreases according to an amount of X-ray absorbed by the foreign matter, an amount of light emission of CsI decreases, and another tube voltage characteristic is obtained. Due to these causes, a change in the tube voltage of the abnormal growth portion appears as a white correction error during white correction. FIG. 15D shows this state. In FIG. 15D, the normal portion has 100/100=1, whereas the abnormal growth portion has 90/80=1.12, which is a white correction error of 12%.
Further, CsI always has a splash portion in vacuum deposition. Current technology has not found any means for eliminating the splash. The splash is a defect caused by the bumping of CsI and has no fixed amount or size. The splash portion has irregular thicknesses and densities. Hence, the splash portion is different from other normal portions in X-ray absorption. Thus, like abnormal growth caused by a foreign matter, white correction causes a white correction error according to a change in the tube voltage characteristic.
Also when a foreign matter enters the substrate 81, a white correction error occurs in the above-described manner. In the above explanation, CsI was discussed as the scintillator 82 of the conventional art. Other scintillators similarly have white correction errors caused by foreign matters.
Moreover, a direct radiation image pick-up device using no scintillator has a similar white correction error. The material of the direct radiation image pick-up device is selected from the group consisting of amorphous selenium, gallium arsenide, mercurous iodide and lead iodide or the like. Like scintillators, a thickness distribution due to the adhesion of a foreign matter occurs during film formation. The generated thickness distribution changes an amount of X-ray absorption like the splash of CsI, causing a white correction error.